The present invention relates to a safety system for determining rotation of a laundry drum of an electric household appliance, in particular a washing machine of the type comprising: a casing, in which the laundry drum is mounted to rotate freely; a door connected to the frame to open and close an access opening to the laundry drum; a three-phase asynchronous motor for rotating the laundry drum; and an inverter, in turn comprising a power circuit composed of six transistors arranged in pairs along three circuit branches connected to the three stator phases of the three-phase asynchronous motor, and a control device that controls the six transistors instant by instant to supply the three stator currents to the motor to generate a rotating magnetic field by which to rotate the rotor.
As is known, washing machine safety systems of the above sort are designed to measure the rotor rotation speed of the three-phase asynchronous motor to determine whether or not the laundry drum is rotating. The information acquired by the safety system relative to rotation or no rotation of the rotor is normally sent to a central control unit which monitors the washing machine and authorizes, or not, safe opening of the door in the event of power failure.
More specifically, in the event of power failure, if the central control unit monitoring the washing machine determines rotation of the laundry drum, it temporarily prevents the laundry drum door from being opened, to prevent the user from accidentally coming into contact with the rotating drum. In fact, the laundry-drum usually has a relatively high value of inertia which causes rotation of the drum for a considerable time interval after a power failure.
For this purpose, some currently marketed safety systems comprise sensors fitted to the motor to measure rotor speed; and a computing module, which determines rotation of the rotor when the speed measured by the sensors is other than zero.
Though efficient and reliable, safety systems of the above type have the drawback of requiring the use of speed sensors, which, besides complicating system hardware, have a far from negligible effect on the overall cost of the safety system.
Accordingly, sensorless solutions have been devised, in which the inverter control device is designed to estimate rotor rotation speed on the basis of the stator currents and voltages, and as a function of a mathematical model of the electric behaviour of the three-phase asynchronous motor.
More specifically, an induction motor can be represented by a system of equations, in which the voltage impressed by the inverter and the motor phase current readings are the inputs, and the rotor speed is the output; and the parameters of the equation are the stator and rotor resistance, and the stator and rotor inductance. Given these parameters, speed can be estimated and implemented in the control device.
Though safely determining rotation or no rotation of the laundry drum on the basis of estimated speed, the above control device is not so reliable in determining rotation or no rotation in the event of power failure.
That is, in the event of power failure, the control device temporarily loses the stator current or voltage references used to drive the motor, and so is unable to make a correct estimate of rotor rotation speed. In which case, the control device resets itself to reset control of the motor, by assuming a stationary-rotor reset condition.
In other words, though efficient and reliable in determining rotation or no rotation of the rotor in “normal” operating conditions, the above control device fails to ensure the same in the event of power failure, thus impairing the safety of the washing machine.